LISST-ABS and Optical Turbidity Views of a River Column

The LISST-ABS would reveal rich sediment structure that turbidity misses.

For measuring suspended sediments, the use of optical backscatter sensors or other turdibity sensors is widespread. At the same time, it is understood that these sensors respond to total particle area, not volume or mass. In other words, optical turbidity sensors have this property: their sensitivity decreases with increasing grain size. The decrease follows 1/diameter. This has been published many times, most notably in an article co-authored by J. Downing, the inventor of OBS. The reason: particles beyond about 0.2 micron in size scatter light following geometric optics (as against ‘Mie’ regime). Here, ka>>1. In contrast, the LISST-ABS operates partly in the Rayleigh zone (ka<<1), but from ~30-400 microns, it is in the transition zone, where ka ~1. In the zone, the scattering strength is relatively independent of grain size (see here, scroll down). This flat response region makes the LISST-ABS a far superior sensor, although not as good as our LISST-100X, or other laser diffraction devices. The figure below illustrates this.

This figure is a composite of actual field data from the Cowlitz River in Washington State, USA, and one panel that is based on calculation. For field data, see here. The first panel is the optical transmission profile (left axis is depth in m). This is like turbidity. It sees no structure that the LISST-SL revealed. The second panel shows the vertical profile of suspended sediment concentration measured by LISST-SL in mass concentration mg/L, and verified with P-61 samples. Obviously, turbidity (first panel) sees none of the structure of this second panel. Now, using the size distribution profile from LISST-SL data, we computed what the LISST-ABS response would be. So, this is a theoretical estimate. This is seen in the third panel. The LISST-ABS sees the structure quite well. Does the LISST-ABS have constant calibration for all size distributions? Not quite. The fourth and last panel shows the ratio of concentration to the ABS output – a calibration. There is variability, though contained in a small zone. This is the key improvement of suspended sediment measurements with the LISST-ABS. Please return to the LISST-ABS product page for subsequent articles.